How to link style.css in functions.php

To get this working:

(1) Your index.php file should begin with:

<?php get_header(); ?>

and end with:

<?php get_footer(); ?>

(2) Your header.php file should have the following right before the closing </head> tag:

<?php wp_head(); ?>

Note: header.php also has other things, but I am assuming you have them set up already.

(3) Your footer.php file should have the following right before the closing </body> tag:

<?php wp_footer(); ?> 

(4) In your functions.php, modify your the second line of your wp_enqueue_style so that it looks like this:

wp_enqueue_style( 'style', get_stylesheet_uri() );

The whole function in the end should look like this:

function mortal_theme() {
    wp_enqueue_style( 'style', get_template_directory_uri() );
  }
add_action( 'wp_enqueue_scripts', 'mortal_theme' );

This is because the style.css in your theme folder is the main stylesheet for the theme – so you don’t need to tell WordPress to look for style.css, it will know what to look for.

Update:

I have recreated your theme using your code on a local WordPress installation to test it, and here is a screenshot of what I see when I visit the page:

The theme folder has the following files:

1. style.css
2. index.php
3. header.php
4. functions.php
5. footer.php

And I copied and pasted the exact code for each file from your original question, with one exception: for functions.php I added the starting <?php tag so my final code looks like this:

<?php
   function mortal_theme() {
      wp_enqueue_style( 'style', get_template_directory_uri() . '/style.css);
   }
   add_action( 'wp_enqueue_scripts', 'mortal_theme' );

Based on the screenshot above, everything seems to be working correctly.

However, if I remove the <?php that I added (to make my code match the one in your question 100%), this is what I see when I view the page:

What do you see when you view your page? If you’re getting results like that in the second screenshot, then it is because you do not have the starting <?php in the beginning of your functions.php file. Add that and you’re good to go, insha’Allah.